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An Experiment to determine Water Potential in Potato Tissue.

Extracts from this document...

Introduction

An Experiment to determine Water Potential in Potato Tissue Introduction Osmosis is the movement of water molecules move from a region of high water concentration (high water potential) to a region of low water concentration (low water potential). The water molecules also move through a partially permeable membrane. Osmosis affects the shape of plant cells. When a plant cell is in a lower water potential (highly concentrated solution), water from the plant cell moves out and therefore it becomes flaccid, and this is when the cell membrane of the plant cell shrinks. When a plant cell is in a solution which has higher water potential, the water moves into the cell and therefore it becomes turgid, this is when the cell membrane expands. In both cases, it is only the cell membrane that changes its shape. The cell wall always keeps its shape. The mass of plant cells change as well. This is because water is moved into or out of the plant cell and therefore a change occurs to its mass. At equilibrium (when the solution's water potential and the plant cell's water potential are the same) there will be no movement of water. Therefore the shape and the mass of plant cell would stay the same. The tendency of water molecules to move from one place to another is measured as the water potential, which is represented by the sign ?. It is a Greek letter for Psi. The water molecules can move because they have kinetic energy and they can move randomly. Solute molecules (molecules which dissolve into liquid to form a solution) always lower the water potential. Pure water has the highest water potential, which is 0?. All the other solutions have negative water potential. Therefore lower the water potential, the solution is more concentrated. Water potential is affected by two main factors: 1. Solute potential 2. Pressure potential. ...read more.

Middle

I also always used the same electronic balance. This was just a precaution in case there was an electronic error (i.e. if the reading shown was more or less than the actual mass.). Firstly I used the cork borers to get the eighteen samples of potato tissue. I then used the scalpel to cut the potato to the required length, which was 25 mm. I used a ceramic tile to ensure there is no damage to the work surface. I put these pieces of potato on a piece of tissue and left them on the side. I then mixed all the solutions, putting the required amounts of sucrose and water. I put all the solutions in individual boiling tubes. Before I put the pieces of potato into the solution I measured the masses of every of the potato. I then put one piece of potato into a test tube, the potatoes stayed in the solution for approximately 20 hours. This is a satisfactory length of time for osmosis to take place, so that a sufficient change in mass takes place. After the 20 hours I removed all the pieces potato from the solution at the same time so it remained a fair test. I then the tissue paper to dry the potato lightly. This is because I had to measure the mass of the potato again and the solution that is left on the potato would increase the mass, thus being an unfair test. I then calculated the change in mass of the potato. The first measurement I took was the mass of the potato before it was put into the solution. After the potato tissue had been in the solution for the required amount of time I measured the mass again. I then subtracted the final mass from the initial mass, to find out the change in mass. All the masses of the potato were measured in grams, to 3 decimal places. ...read more.

Conclusion

Getting the right ratio of distilled water and sucrose solution was very difficult. This is because measuring cylinder scales were very narrow and getting the right ratio was hard. Also if you put in too much or too small, it was hard to sort out as you don't know whether to add more distilled water or sucrose solution. I think that the biggest problem was drying the soaked potato pieces before weighing. The problem is that I might have dried one potato piece much more than the another piece. This defeats the whole object of this experiment in which you are meant to find out the mass after water molecules have been moved around. To insure that this wouldn't be a problem, I think tissue shouldn't be used to dry however appropriate drying equipment should be used. I think my results were pretty reliable. My repeated results were very similar for nearly all of them. They are all very close e.g. on concentration 0.4 moldm-3; the max difference was 0.04 grams, which is very little. Also on concentration 0.2 moldm-3, maximum difference was 0.05 grams, which is even more accurate. This proves that my results were very reliable. To improve the reliability, I would do more repeats. This will ensure that reliability of results are very high as if there is a faulty result, it would stand out clearly if there are more repeats. Also doing more concentrations and narrowing the scale of concentration would improve reliability. (i.e. doing 0.1, 0.15, 0.2, 0.25, etc). Also using more accurate measuring equipment would have increased reliability, especially for measuring length. I could have used different methods. Instead of measuring the change in mass, I could have measured the volume. Therefore in this experiment, if the volume stays the same, that would be the water potential of potato. I think in some ways this could be more accurate because no tissue is needed which caused most problems for my experiment when I was measuring the mass. However, measuring the volume would be very difficult. Rikin Patel Water Potential Coarsework.doc Page 1 08/05/2007 ...read more.

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Here's what a teacher thought of this essay

4 star(s)

**** Overall a sound experimental report that demonstrates a thorough experimental approach to calculating the water potential of potato tuber cells.

To improve:
Planning:
The candidate produced a testable hypothesis backed up some relevant biological theory although there were some inaccuracies in the explanations. It would help to include more selected background reference material to help inform the plan. This should be referenced. The readability could be improved if some of the key information was in the form of tables rather than long paragraphs of text.
Carrying out:
A trial experiment was used to help inform the plan and a good range of the independent variable was selected. Tables could be improved by ensuring full details are included in the headings.
Analysis and Evaluation:
The graph was discussed but was not included. The discussion of the results was thorough and was explained using relevant biological theory. The writer tried to explain anomalous results. The evaluation could have been improved by making the suggestions for improving the method or range of the work more specific. For example the suggestion to use more accurate equipment should have included the instruments to use for this task.

Marked by teacher Stevie Fleming 29/05/2013

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